Learn Hardcore Japanese: tomodati ha taisetu desu.

Japanese often omits subjects when they’re understood from context. Here the statement “Friends are important” is a general truth, so there’s no need to say 私は or 僕は. If you want to specify “my friends,” you would say 私の友達は大切です without adding another 私は.

What is the function of は in 友達は?

は is the topic‐marker particle. It marks 友達 (“friends”) as the topic you’re talking about—literally “as for friends….” It does not directly translate to “is,” but signals what the sentence will comment on.

What part of speech is 大切, and why isn’t there a な before です?

大切 (たいせつ) is a な-adjective (sometimes called an adjectival noun). When linking a な-adjective to a copula (です/だ), you do not insert な. You only use な when the adjective directly modifies a noun (e.g. 大切な本 “an important book”).

What does です do, and why is it used here?

です is the polite copula (linking verb), similar to English “is/am/are” but with politeness built in. It completes the sentence in a polite register: “Friends are important.” In casual speech you’d swap it for だ (see next question).

Why is there no plural marker on 友達? How do we know it’s plural?

Japanese nouns don’t change form for singular vs. plural. 友達 can mean “friend” or “friends” depending on context. Here, because it’s a general statement about friends in general, it’s understood as plural.

How would I say this more casually?

– Swap です for the plain copula だ:
友達は大切だ。
– In very casual speech you can even drop だ (especially in conversation):
友達は大切。

How can I specify “my friends” instead of just “friends”?

Use the possessive particle の with 私:
私の友達は大切です。
You can omit 私は because the possessive phrase already shows whose friends they are.

What’s the difference between 友達は大切です and 友達が大切です?

– は marks 友達 as the general topic: “(As for) friends, they are important.”
– が marks 友達 as the grammatical subject, often to single it out or contrast it: “It is friends that are important (not something else).”
In neutral, everyday statements you’ll almost always see 友達は大切です.